1. Field of the Invention
The present invention relates to detecting no load conditions in a switching power converter and reducing the target regulated output voltage of the switching power converter or shutting down the switching power converter under the detected no-load conditions.
2. Description of the Related Arts
Switching power converters are often used in battery chargers for electronic devices such as cellular telephones and laptop computers to convert AC input voltage to a regulated DC voltage for use in charging the batteries. Regulation of the output voltage of switching power converters is generally accomplished by sensing the difference between an output voltage feedback signal approximating the output voltage and a reference, and using such difference (or error voltage) to determine how to control the on and off states of the power switch in the switching power converter so as to minimize the difference between the output voltage feedback signal and the reference. There are many well-known modulation techniques that can be used to control the ON and OFF cycles of the power switch of the switching power converter in order to maintain output regulation, including pulse-width-modulation (PWM) and pulse-frequency-modulation (PFM). The output voltage feedback signal can be obtained by directly sensing the output voltage of the switching power converter, or by indirectly sensing a signal that represents the regulated output voltage level. Indirect sensing can be especially useful in isolated power converters such as a flyback type switching power converter, where the controller is typically located on the primary side of a transformer in the switching power supply and the feedback signal can be provided by sensing a signal on the primary side of the transformer.
In general, battery chargers maintain the regulated output voltage regardless of whether the electronic device is connected to the battery charger. That is, battery chargers would provide the constant regulated output voltage even while the electronic device is not connected to the battery charger, so that the battery charger may be ready for the electronic device to be connected to the battery charger for charging. Although there is no energy delivered to the load during these periods of no-load conditions where the electronic device is not connected to the battery charger, there is still energy, often referred to as standby power, that is consumed by the switching power converter in order to maintain the regulated output voltage. Because of the large number of battery chargers that are used to charge the countless number of portable electronic devices, standby power loss may add up to a large amount of waste of energy.